fix(web): unify neutral glass colors and white filled controls

[Qiyuanqiii]

Summary

V2 defined its own purple accent (#6b55ed) and several components supplied independent orange, yellow, blue and green fills. These bypassed the shared theme and made ordinary controls, recovery states and provider cards look unrelated. This follow-up to #294 gives legacy and V2 screens one neutral light/dark/glass palette, opaque white primary controls with dark text, and high-opacity glass surfaces.

• Remove decorative color gradients, tinted shadows, literal component fallbacks and unused provider accent metadata. Preserve shared error/destructive colors, code-diff colors and terminal ANSI content.
• Make reduced-transparency surfaces opaque with blur disabled, including the glass-theme selector precedence fix found in the browser. Keep keyboard focus, increased contrast and forced colors legible.
• Record the inventory and material conventions in docs/branding/neutral-glass-palette.md. Across 24 CSS files, authored color values decrease from 521 to 93; V2's 6 purple and 24 yellow/orange/brown values are removed. These counts describe source definitions, including historical rules and alpha variants.

Validation

• Frontend suite: 186 files / 1,834 tests passed; final model-card follow-up: 6 tests passed.
• TypeScript and production build passed; the existing large-chunk advisory remains.
• All 24 CSS files parsed with PostCSS; git diff --check passed.
• Windows portable build at f5d1f859: desktop/web boundary and Wails adapter tests passed; two consecutive executable builds produced the same SHA-256. Automated compatibility checks passed; the manual Windows 10 matrix remains open.
• Real local Go API and isolated fixture database: light/dark/glass, 1280 px and 390 px layouts, keyboard focus, reduced transparency, increased contrast, forced colors and permission-menu dismissal checked in the browser. Original screenshots and computed-style evidence are retained locally.
• go test -count=1 ./... — not rerun locally for this frontend-only change.
• go vet ./... — not rerun locally for this frontend-only change.
• Native Windows Acrylic/wallpaper compositing remains a separate visual check.

Surface governance

• No Surface is added, promoted, downgraded, deprecated, or removed.
• Registry item(s), target tier / transition, entry criteria / decision, owner, shared Go Application contract, authority impact, supported platforms, release / test evidence, compatibility strategy, deprecation window, removal / rollback plan: N/A — no Surface change.

Audit

• No credentials or local runtime data are included.
• Policy, workspace, sandbox, and persistence boundaries were reviewed; their behavior is unchanged.
• Project memory, frontend status and theme documentation were updated.