Settings: Tab width, Dynamic (default) or Fixed, first on Appearance (#56)

[Maxaubert]

Closes #56. Owner, 2026-09-23: "add a setting for tab width, where the user can pick fixed size or dynamic, so essentially what we got now and what we had before"; then "put the option closer to the top of appearance, and call it dynamic not based on name. also have dynamic be the default setting, not fixed".

• Settings > Appearance opens with Tab width: Dynamic (the DEFAULT: each tab as wide as its name, capped at 14rem, shrinking only when the strip runs out of room, the strip from before Look: the app icon, Pitch and Cinder in its orange, and every tab one width #35) or Fixed (every tab 114px, all shrinking equally, as since Look: the app icon, Pitch and Cinder in its orange, and every tab one width #35).
• Default is Dynamic by the owner's word, so everyone's strip goes back to dynamic with this update.
• The row carries its own top rule and drops its bottom one, so the line above the theme block is not doubled (screenshot looked at).
• Stored as prism.window.tabWidth (lib/tabWidthPrefs.ts), read defensively (anything unknown is Dynamic). This app's own row, like Edges; in the options e2e's list of this app's rows.
• Prism gets the same row at the top of its Style page in its own PR.

App 0.15.0 -> 0.16.0 (no core change).

Tests

• Unit: tabWidthPrefs.test.ts (default Dynamic, both values, unknown values, listeners).
• e2e tabWidth: by default a long name makes a wider tab than a short one (72 / 267 / 66px) and the long one stops at the cap, truncated; Tab width is the first row of Appearance; picking Fixed stores it and gives every tab 114px, and picking a tab then moves none. Screenshots .e2e-shots/tabs-dynamic.png, tabs-fixed.png, settings-tab-width.png, looked at.
• 886 unit, typecheck, lint 0 errors, full e2e: all scenarios passed.

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