…n, flat↔gradient conversion, frameless chrome

## Summary

Closes the last visible gap from RFC #3076's Phase 5 — every theme
token can now be edited end-to-end through a frameless dialog flow.
Adds a gradient stop-strip editor, makes any colour token convertible
between flat-fill and gradient form, tokenises the level-meter fill
through a new shared gradient, and converts both editor dialogs to
the canonical `PersistentDialog` chrome shape.

## GradientEditorDialog

New `src/gui/GradientEditorDialog.{h,cpp}` — modal dialog with a
custom `GradientStrip` widget driving the interaction:

  * Live gradient preview over a checkerboard so translucent stops
    stay visible.
  * Triangle markers below the strip, draggable to reposition.
  * Double-click strip body → add stop (colour sampled from the
    gradient at the click point so the addition splits the ramp
    visually).
  * Right-click marker → delete stop (refuses below 2 stops).
  * QListWidget alongside with explicit `+`, `−`, `Edit colour…`
    buttons for keyboard-driven editing.
  * Angle spinbox for linear gradients (radial defers angle).
  * `Reset to default` button (`QDialogButtonBox::RestoreDefaults`)
    pulls the canonical value from `:/themes/default-dark.json` via
    a new lazy-loaded `ThemeManager::factoryGradient()` snapshot.
    Disabled when the token has no factory baseline.

## ThemeManager API additions

  * `ThemeGradient gradient(token)` / `setGradient(token, g)` — typed
    accessors paralleling the scalar `color()` / `setColor()` path.
    `setGradient` overwrites any prior scalar entry; `setColor`
    likewise overwrites a prior gradient.  Conversion at the data
    layer is therefore free — UI just routes through the right setter.
  * `factoryGradient(token)` — reads from a one-shot snapshot of the
    bundled `:/themes/default-dark.json`.  Lazy-loaded so a missing
    resource bundle doesn't crash the singleton.

## ThemeEditorDialog wiring

  * Token list now shows gradient rows with a mini left-to-right
    preview swatch + descriptor (`linear, 0°, 5 stops`).  Scalar rows
    keep their hex label + checkerboarded swatch from PR #3148.
  * Clicking any row pops a small `QMenu` at the cursor with two
    options that always reflect the current type:
      - "Edit flat colour…" + "Convert to gradient…" for scalars
      - "Edit gradient…" + "Convert to flat colour…" for gradients
    Picking "Convert to gradient" seeds a 2-stop gradient with the
    scalar colour at both ends — initial render is bit-identical to
    the flat output so opening the editor doesn't perturb anything
    until the operator makes a deliberate edit.  Picking "Convert
    to flat" seeds `QColorDialog` with the gradient's first stop.

## ThemeInspector — pause during modal child dialogs

While the editor has spawned a modal child (`QColorDialog`,
`GradientEditorDialog`, "Save As" confirms), the inspector's global
event filter now early-returns and hides its overlay.  Without this,
the cyan tracking outline draws on top of the modal dialog because
the modal isn't recognised as a descendant of the editor's
`isAncestorOf()` chain (top-level windows don't traverse).

## Meter-bar gradient

  * New token `color.meter.bar.fillGradient` — linear, angle 0°
    (bottom→top), 5 stops: green / lime / amber / red / deep-red.
    Matches the original hardcoded palette so existing installs see
    no visual diff.
  * `ClientLevelMeter` reads the gradient via
    `ThemeManager::brush(token, stripBox)`, dropping its hand-rolled
    `QLinearGradient` block.
  * `ClientCompMeter` switches to the same shared token, replacing
    its `kLevelLo`/`kLevelMid`/`kLevelHi` 3-stop assembly.  Both
    meters now recolour from one place — themes can edit the
    gradient once and every level-style meter follows.
  * Both widgets connect to `themeChanged` for live updates.

## Frameless chrome

Per `docs/style/dialog-patterns.md`, both editors now subclass
`PersistentDialog`:

  * `ThemeEditorDialog` — frameless title bar, 8-axis edge resize,
    geometry persists under `ThemeEditorDialogGeometry`.  MainWindow
    wireup collapsed from a 10-line lazy-construct block to a single
    `showOrRaisePersistent(m_themeEditorDialog)` call; member changed
    to `QPointer<ThemeEditorDialog>`.
  * `GradientEditorDialog` — same chrome; inherits frameless state
    from `AppSettings` at construction.  Still modal so the parent's
    `dlg.exec()` flow is preserved.

## Test plan

- [x] Build clean (`--target all`)
- [x] Click `color.waterfall.colormap.fire` → "Edit gradient…" → strip
      editor opens; drag/add/delete stops repaint the panadapter
      waterfall live; "Reset to default" restores the bundled stops
- [x] Click `color.accent` → "Convert to gradient…" → strip editor
      opens seeded with a 2-stop same-colour gradient; making edits
      flows to every applyStyleSheet-tracked widget
- [x] Click `color.meter.bar.fillGradient` → gradient editor; edits
      recolour the slice level meters AND compression meters together
- [x] Inspector mode + open gradient editor → overlay hides while
      modal is up, resumes on close
- [x] Theme Editor frameless chrome matches the rest of the dialogs
      (title bar, drag-to-move, edge resize, geometry persists)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>