fix(components): use a touch slider for conversation font size

[lodystage]

Risk: 🟢 low | Confidence: high — isolated appearance control; covered by component tests and browser touch/keyboard checks.

Problem / pressure

Adjusting conversation text size on a phone should be a direct touch interaction. The existing size picker requires opening a menu for each adjustment.

Summary

• Replace desktop and mobile size pickers with one native slider, a pixel readout, and a 44px touch area. Mobile gives the control its own row.
• Retain the eight sizes and persistence normalization introduced in fix(components): pick the conversation font size from a fixed scale #718, and update the existing sample text as the slider moves.
• Cover boundaries, persistence after remount, live sample rendering, and light/dark visual states. Remove the unused picker-option helper.

Visual explanation

Mobile / desktop appearance
  → native range index (0–7)
  → CONVERSATION_FONT_SIZES[index] (pixels)
  → existing setting state → saved preference + readout + sample text

Phone screenshots were shared in the task conversation.

Before / after

Before	After
Open a size menu, then select an option	Drag directly through the same eight sizes
Desktop hover temporarily previews a menu option	Dragging immediately saves the size and updates the sample

Test plan

• Appearance and font-setting suites: 11 tests passed.
• Browser checks: touch drag, live readout, persisted reload, keyboard bounds/steps, 320px and 393px layouts, and desktop pointer drag passed.
• Storybook screenshots inspected in light/dark mode; not a packaged native-app test.
• Full OSS check: typechecks and lint passed; components passed 477 files / 3671 tests. CLI had one macOS /var versus /private/var path assertion failure; its 11-test suite passed with TMPDIR=/private/tmp. All other CLI tests passed. The interrupted remainder was run separately: Electron (112 tests), i18n, import/platform/public-boundary guards all passed.
• Formatting and documentation checks passed. The private affected-check wrapper is unavailable in this public checkout (missing scripts/format-check-changed.mjs), so the public repository's full check was used.

Context handoff

Instructions for reviewing agents

• Review focus: Slider indices must map to CONVERSATION_FONT_SIZES; storage and sample rendering must receive pixel values.
• Decisions to challenge: Immediate persistence while dragging, full-width mobile layout, and native range accessibility.
• Plausible failures / evidence gaps: Native touch layout was previewed, but no new iOS binary was packaged.