design-system: extract shared model-load progress panel (LoadingPanel dup between RewriteButton + SectionRewrite)

[Vaishnavi1709]

Background

Two feature components hand-roll the same WebLLM model-download progress panel — same chrome, same WebGPU progress bar, same "what's happening" disclosure copy. Only the headline label string differs:

• src/components/features/RewriteButton.tsx — LoadingPanel at line 193. Label: "Loading the bullet-rewrite model (~1.2GB, one-time download)".
• src/components/features/SectionRewrite.tsx — LoadingPanel at line 244. Label: "Loading the rewrite model (~1.2GB, one-time download)".

Both render: an outer <div> with border-border-light / bg-surface-subtle chrome, a header row with the label + pct%, a role="progressbar" track + fill, an optional progress-text line in monospace, and (in RewriteButton's case) the "What's happening?" <details> disclosure that explains the model is downloading locally.

This violates CLAUDE.md's reuse rule:

Never hand-roll a parallel copy or an inline one-off. If a shared piece is missing a variant, add the variant to the shared piece.

Pre-existing duplication — shipped in #123 (PR #123) and untouched by PR #129. Flagged in #129's review pass but explicitly deferred as out of scope for #64 PR A. This issue is that follow-up.

Proposed surface

New shared component at src/design-system/shared/ModelLoadProgress.tsx:

interface ModelLoadProgressProps {
  /** 0..1 fraction reported by WebLLM. */
  progress: number;
  /** Human-readable status from WebLLM (e.g. the weight file being fetched). */
  text?: string;
  /** Headline label. Defaults to "Loading the rewrite model (~1.2GB, one-time download)". */
  label?: string;
  /** Show the "What's happening?" disclosure (`RewriteButton` does; `SectionRewrite` currently doesn't). */
  showExplainer?: boolean;
}

Re-export from the @design-system barrel. RewriteButton.LoadingPanel and SectionRewrite.LoadingPanel both delete their local copy and use this.

Pros: one canonical model-download panel; the "What's happening?" disclosure can be enabled in SectionRewrite too without a second hand-roll; future model-load surfaces (PR B's ModelSelector will need this too — same progress bar, switching to a different model) reuse it.

Cons: minor — design-system needs a small new export; consumers thread two extra optional props.

Relationship to #124

#124 tracks the OTHER duplication between these two files — the success/warning result strip (RewriteResult in RewriteButton, ProposedSection in SectionRewrite). Different chrome, different purpose. Both should land before the Dialog primitive in PR B of #64 so the new surfaces don't add a third copy of either pattern.

Acceptance criteria

• New shared component at src/design-system/shared/ModelLoadProgress.tsx.
• Re-exported from the @design-system barrel (src/design-system/index.ts).
• RewriteButton.LoadingPanel removed; consumer imports the shared component.
• SectionRewrite.LoadingPanel removed; consumer imports the shared component.
• Default label preserved per-call site so existing copy ("bullet-rewrite model" vs "rewrite model") doesn't change unless intentionally consolidated.
• No visual regression — npm run dev, both rewrite paths look identical to current.
• npm run test, npm run typecheck, npm run build green.

Out of scope

• Removing the ~1.2GB literal in the label string (size depends on which model is loading; will be parameterized when PR B ships the model-dimensioned label).
• Consolidating with the success/warning result strip (that's design-system: extract shared inline-result chrome (Card variant vs InlineResult primitive) #124).

Follow-up to PR #123 + PR #129.