feat(settings): add extra-large font size option for tablet and desktop readability

[franksong2702]

Problem

The Appearance settings currently expose Small, Default, and Large font sizes. Large helps, but it is still too small for some tablet and large-desktop usage, especially when the user is reading long sessions from farther away.

Proposed quick fix

Add a fourth explicit font size option:

• Small
• Default
• Large
• Extra Large

This should be a user-controlled Appearance preference, not automatic device detection.

Scope

• Add the new setting value to validation/persistence paths.
• Add the Appearance UI option and localization key.
• Add CSS sizing hooks for the extra-large mode.
• Add regression/static coverage so the option does not appear only in the UI without being accepted by settings boot/apply paths.
• Update the changelog.

Non-goals

• No mobile layout redesign.
• No automatic viewport-based font scaling.
• No changes to the existing Small, Default, or Large meanings.
• No broader Appearance settings redesign.

[franksong2702]

Opened PR #2225 for the focused quick fix: add an explicit Extra Large Appearance font-size option.

Scope is intentionally narrow:

• accepts the new persisted xlarge setting value
• adds the fourth Appearance picker option
• adds targeted CSS scaling for the existing font-size-controlled surfaces
• adds locale keys, changelog entry, regression coverage, and before/after screenshots

It does not change the default font size, add automatic viewport detection, or redesign the broader mobile layout.

[nesquena-hermes]

Accepting as a sprint-candidate, Milestone 2. Direction:

• New token: --font-scale-xl: 1.25 (or similar — pick a value that's perceptibly larger than the current Large without breaking the existing line-height grid).
• Settings → Appearance: add Extra Large after Large in the radio/select.
• Validation: extend the existing whitelist in the settings serializer to accept xl (or whatever the persisted value is).
• Localization: add the new label to en.json first; backfill locales in a follow-up (don't block).
• CSS: add the matching [data-font-scale="xl"] rule with the new scale.
• Static-coverage regression test: assert the new option round-trips through the boot/apply path (the existing test for Small/Default/Large should be parameterized to include the new value).

Probably ≤60 LOC + 1 test. Keep the existing Small/Default/Large values byte-identical so users with persisted preferences don't get migrated.

Direct contributor pickup welcome.