Companion: rules-based, context-aware writing advice

Replace the kitten's random generic tips with a deterministic, LLM-free
prose checker that reads the actual document and surfaces one gentle,
bilingual note at a time — quoting a slice of her own sentence so the
advice is clearly about *this* draft.

Rules (all conservative, tuned for a Mandarin-native ESL writer):
run-ons, comma splices, unclear antecedents, Oxford comma, a/an,
uncountable plurals, capitalize languages/days/months, subject-verb
agreement, plural-after-number, double determiner, "there is" + plural,
its/it's, than/then, comma-after-transition, doubled words, lowercase
"i", sentence capitals, and spacing around punctuation.

Each rule is paired with false-positive guards (e.g. "a university",
"After dinner, I went home and read", existing Oxford lists) and pinned
by a 45-case Vitest suite (npm test).

Also: bubbles now linger proportional to how much there is to read
(zh+en) and pause-on-hover, so denser advice no longer vanishes mid-read.

Claude-Session: https://claude.ai/code/session_016Yr6jELuRc7hyzYLccQKZd

web/src/components/Companion/tips.ts

@@ -17,7 +17,8 @@ export const ENCOURAGEMENTS: Line[] = [
   { zh: '嗯嗯，这样清楚多了 👍', en: 'Mm, that’s much clearer.' },
 ]
 
-// Gentle, periodic writing/ESL tips. Surfaced only when nothing else is showing.
+// Gentle, generic writing/ESL tips — the fallback when the rules-based prose
+// checker (prose.ts) finds nothing concrete to point at in her current text.
 export const TIPS: Line[] = [
   { zh: '小贴士：英文句子短一点，会更清楚哦。', en: 'Tip: shorter English sentences often read clearer.' },
   { zh: '别忘了冠词 “the” 和 “a” 哦。', en: "Don't forget articles like “the” and “a”." },