Tier 2: Rename Letters 'Refine' paragraph action to 'Polish' (copy only, deferred)

[NickCrew]

Summary

Rename the Letters paragraph "Refine" action to "Polish" — a Letters-local
label/copy change. Tier 2, explicitly deferred in
docs/development/design/ai-action-verbs-and-sharpen-mechanic.md
(Status: Proposed, 2026-06-21).

Why

The gem metaphor drives the verbs (the product is Facet; facets are cut gem
faces): Sharpen cuts the facets across the whole artifact; Polish brings up
the shine on one element. "Refine" reads as metallurgy — refining ore — the
wrong mineral. Polish is the right verb for the localized, user-feedback-driven,
single-paragraph edit.

This is a copy/label change only — the mechanic (refineCoverLetterParagraph,
the refinement notes field, single-element user-feedback edit) stays as-is.

Call sites (grounded 2026-06-21)

• src/routes/letters/LettersPage.tsx:1230 — button label "Refine Paragraph"
  / loading "Refining..." (also currently a RefreshCw icon — icon choice is
  Tier-1 Standardize AI-action button styling and labels site-wide #31's concern, not this rename).
• src/routes/letters/LettersPage.tsx:1208 — "Refinement notes" label.
• src/routes/letters/LettersPage.tsx:1204 — caption copy "AI refinement".

Internal identifiers (refineCoverLetterParagraph, refinement?: string,
CoverLetterParagraphRefinementRequest, handleRefineParagraph) are out of
scope — this is user-facing copy only, to avoid churning the persisted
refinement field name and its validation/migration surface.

Scope

• User-visible Letters strings: "Refine Paragraph" → "Polish Paragraph",
  "Refining..." → "Polishing...", "Refinement notes" → "Polish notes", caption copy.

Out of scope

• Renaming the persisted refinement field or the generator function.
• Icon standardization (Tier 1, Standardize AI-action button styling and labels site-wide #31).

Acceptance criteria

• No user-facing "Refine"/"Refinement" copy remains on the Letters paragraph
  action; it reads "Polish".
• The persisted refinement field and generator API are untouched.
• LettersPage tests updated for the new copy.

Design: docs/development/design/ai-action-verbs-and-sharpen-mechanic.md (verb table; "deferred rename of Refine")
Related: #31 (Tier 1)