refactor(theme): build the font picker from the shared picker parts

[kipavy]

The font picker added in #201 came with its own trigger, rows, dividers, footer
actions and search box. That left the one dropdown sitting next to the appearance
settings looking unlike every other dropdown in the app: a smaller trigger whose
chevron swapped glyphs instead of rotating, rows tinted with accent-12% instead of
carrying an accent label, and its own radius, paddings and shadow.

It is now assembled from PickerTrigger, PickerSurface, PickerOption,
PickerDivider and PickerFooterAction — the same parts as the encoding, key,
identity and folder pickers.

That is not only cosmetic. Going through PickerSurface gives the list the
surface's behaviour, which the in-flow absolute menu could not have:

• it is a portalled float that flips above the trigger when the theme panel is
  scrolled near the bottom, instead of opening down into the scroll region;
• it is a bottom sheet on Android, where the theme creator previously showed a
  desktop-sized float.

Two primitives are widened, both reusable:

• PickerTrigger and PickerOption take an optional labelFont, so a row can
  preview itself in the face it names;
• PickerOption's icon becomes optional, for rows whose label is the whole
  content — the shape DropdownMenuItem already allowed.

The in-surface filter box was copied byte-for-byte between the distro icon picker
and the host command field. It is extracted as PickerSearch and used by all
three, so this removes a duplicate rather than adding a third.

One layout fix came out of looking at it: a nested fixed-height scroller inside
PickerSurface fights the surface's own 320px cap — two scrollbars, and a row
clipped mid-glyph. The surface is the only scroller now, with the filter pinned
sticky top-0.

Font previews, the monospace filter and its "show all fonts" escape hatch, the
free-text box and the "not installed" warning all behave as before.

Verification

tsc --noEmit clean; 207 tests across shared, connections and settings pass.

Driven in the headless build and read back from screenshots:

• app font dropdown — presets, divider, pinned filter, installed families each
  rendered in their own face, accent check on the selection;
• terminal font dropdown — only *Mono families listed, Show all fonts and
  Custom font… footer actions;
• typing an unavailable family into the custom box — warning triangle in the
  trigger plus the "isn't installed" line;
• distro icon picker — search intact after the PickerSearch extraction.