Phase 19: the copy stops being hardcoded Mandarin

Every `中文 · English` string moves out of ~29 components into
web/src/i18n: one Pack type, a verbatim zh pack, and two ways to read
it — usePack() for components, pack() for the modules that build a line
when something happens rather than when something renders.

Anything with a value in it is a function on the pack rather than a
template at the call site, English pluralisation included: word order
isn't universal, and a pack author has to be able to move the number.
The roster constants (tones, rewrite styles, export formats, companions)
keep only value + emoji, so a label can't drift from its key.

On the server, internal/llm/lang.go replaces "Simplified Chinese" in the
three prompts that actually name her language. pt-PT is spelled
"European Portuguese (pt-PT, never Brazilian Portuguese)" in the prompt
itself, and each Lang carries her word for "why" so the tutor prompt
still recognises the question when she asks it her way.

pair_lang reaches the model through the row-scoped query each handler
already ran — the one that proves she owns the document — rather than a
second lookup that could disagree with it.

Also records Phase 18's deploy: migration 0011 rehearsed against a copy
of the live VPS database, then applied for real.

web/src/i18n/i18n.test.ts

@@ -0,0 +1,122 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+
+import { onPackChange, pack, resetPackForTests, setPackLang } from './index'
+import { zh } from './packs/zh'
+
+beforeEach(() => {
+  resetPackForTests()
+})
+
+describe('pack selection', () => {
+  it('speaks the default pair before /api/me answers', () => {
+    // Modules that build copy at import time (the companion, the prose checker)
+    // read the pack before the session is known. That read must not be blank.
+    expect(pack()).toBe(zh)
+    expect(pack().code).toBe('zh')
+  })
+
+  it('falls back rather than blanking on a pair with no pack yet', () => {
+    // A pair_lang the deployment has no copy for is a deployment that got ahead
+    // of its translation. She should still get a working editor.
+    setPackLang('pt-PT')
+    expect(pack()).toBe(zh)
+    setPackLang('klingon')
+    expect(pack()).toBe(zh)
+    setPackLang('')
+    expect(pack()).toBe(zh)
+    setPackLang(null)
+    expect(pack()).toBe(zh)
+    setPackLang(undefined)
+    expect(pack()).toBe(zh)
+  })
+
+  // Only one pack ships today, so a *real* switch can't be exercised yet; what
+  // can be is the other half of that contract — that a no-op never wakes every
+  // reader in the app. The switching path gets its test when pt-PT lands.
+  it('never notifies readers when nothing actually changed', () => {
+    const seen = vi.fn()
+    onPackChange(seen)
+
+    setPackLang('zh') // already the current pack — nothing changed
+    expect(seen).not.toHaveBeenCalled()
+
+    // Standing in for a second pack until one ships: switching to something
+    // unshipped resolves back to zh, which is also not a change.
+    setPackLang('fr')
+    expect(seen).not.toHaveBeenCalled()
+  })
+
+  it('stops notifying after unsubscribe', () => {
+    const seen = vi.fn()
+    const off = onPackChange(seen)
+    off()
+    setPackLang('zh')
+    expect(seen).not.toHaveBeenCalled()
+  })
+})
+
+describe('the zh pack', () => {
+  // Sentinels: a handful of strings copied from the pre-Phase-19 source. The
+  // point of the extraction was that nothing she reads changed, and a reworded
+  // label would otherwise be invisible in a diff of this size.
+  it('carries the original copy verbatim', () => {
+    expect(zh.docs.searchPlaceholder).toBe('搜索 · Search')
+    expect(zh.editor.addToDictionary).toBe('添加到词典 · Add to dictionary')
+    expect(zh.status.savedLocally).toBe('已保存在本机 · Kept on this device')
+    expect(zh.garden.titleWithFlower).toBe('🌷 词汇花园 · Vocabulary Garden')
+    expect(zh.companion.greeting).toEqual({
+      native: '嗨～我在这儿陪你写作哦 🐱',
+      en: "Hi! I'm right here keeping you company. 🐱",
+    })
+    expect(zh.prose.itsOwn).toBe('“it’s” = “it is”；表示“它的”要用 “its”，所以是 “its own”。')
+  })
+
+  it('renders its interpolated lines with the value in place', () => {
+    expect(zh.app.duplicateTitle('Spring')).toBe('Spring (副本)')
+    expect(zh.companion.milestone(300).native).toBe('哇！已经 300 个词了，太厉害了 🎉')
+    expect(zh.prose.articleAn('apple')).toContain('“an apple”')
+    expect(zh.prose.uncountable('informations', 'information')).toContain('“information”')
+    expect(zh.garden.reviewDue(4)).toBe('复习 4 个词 · Review 4 due 🌸')
+    // English pluralisation is the pack's job, not the call site's.
+    expect(zh.garden.growing(1)).toContain('1 blossom growing')
+    expect(zh.garden.growing(2)).toContain('2 blossoms growing')
+    expect(zh.history.daysAgo(1)).toBe('1 day ago · 1 天前')
+    expect(zh.history.daysAgo(3)).toBe('3 days ago · 3 天前')
+  })
+
+  // A pack with a hole in it renders an empty label rather than failing, which
+  // is exactly the kind of thing that reaches production. Types catch a missing
+  // *key*; only this catches an empty *value*.
+  it('has no empty strings anywhere', () => {
+    const empties: string[] = []
+    const walk = (node: unknown, path: string) => {
+      if (typeof node === 'string') {
+        if (node.trim() === '') empties.push(path)
+        return
+      }
+      if (typeof node === 'function') return // exercised above
+      if (node && typeof node === 'object') {
+        for (const [k, v] of Object.entries(node)) walk(v, path ? `${path}.${k}` : k)
+      }
+    }
+    walk(zh, '')
+    expect(empties).toEqual([])
+  })
+
+  it('labels every companion in the roster and every tone the editor offers', async () => {
+    const { COMPANIONS } = await import('../components/Companion/companions')
+    for (const c of COMPANIONS) {
+      expect(zh.companion.names[c.id], `no name for companion ${c.id}`).toBeTruthy()
+    }
+
+    const { TONES } = await import('../components/Editor/ToneSelect')
+    for (const tone of TONES) {
+      expect(zh.tones[tone.value], `no label for tone ${tone.value}`).toBeTruthy()
+    }
+
+    const { REWRITE_STYLES } = await import('../components/Editor/SelectionBubble')
+    for (const style of REWRITE_STYLES) {
+      expect(zh.styles[style.value], `no label for style ${style.value}`).toBeTruthy()
+    }
+  })
+})