reco-gui: flat control language restyle

[RufanMelfor]

One consistent flat visual language across the toolbar and panels,
replacing std-widgets Button and the earlier ad-hoc SectionHeader.

• New FlatButton component (28px, 7px radius, flat fill + hover
  lighten, blue primary variant) replaces all std-widgets Button
  usages across the toolbar, panels, and dialogs.
• New TransportButton component for the tight playback-controls
  cluster (jump/step/play icons); the play/pause button gets an
  emphasized (accent-tinted, wider) variant.
• SectionHeader rebuilt: a filled yellow square when expanded
  (visible at a glance across a stack of collapsed sections) plus a
  ›/⌄ chevron, instead of a plain ▸/▾ text glyph.
• Color tokens re-based to cool-biased dark neutrals (a hint of blue
  toward the cyan accent); accent green brightened. Light-mode values
  are unchanged.
• SegmentList file rows: taller, bordered, more rounded; red hover
  on the remove ×.
• New FOV pill overlaid top-center on the preview (FOV 75° · Constrained), non-interactive so pan/zoom drag underneath still
  works.
• Fixed the toolbar's floating panel-toggle: it was a bare Rectangle
  with no x, sibling to the toolbar's HorizontalBox - Slint
  centers an unpositioned element inside its parent, so the ◀/▶ glyph
  floated in the middle of the toolbar instead of sitting at the far
  right. Moved into the toolbar row's own flow as a proper icon button
  (⇤/⇥).

Pure visual/styling change - no new functionality, no property or
callback signature changes, no Rust changes.

This is a deliberately scoped-down extraction: the original fork
implementation bundled this restyle together with 7 unrelated
functional features (sync-offset auto-detection, variable playback
speed, ground/top-plane tilt sliders, etc.) in one commit. Splitting it
out cleanly meant re-deriving the visual-only pieces against current
main, rather than cherry-picking.

Test plan

• cargo check -p reco-gui --all-targets clean
• cargo test -p reco-gui — 6/6 passing (no test coverage change,
  this PR touches only .slint)
• cargo clippy -p reco-gui --all-targets -- -D warnings — no new
  findings; failures shown are the same pre-existing reco-core
  dead-code/unsafe-ptr issues already tracked by Clear pre-existing clippy -D warnings failures on Windows #423
• Manually reviewed every converted Button → FlatButton call
  site for property compatibility (text/enabled/clicked only
  - no usages relied on Button-only features like checkable)

[github-actions]

All contributors have signed the CLA. Thank you!
_{Posted by the CLA Assistant Lite bot.}

[RufanMelfor]

I have read the CLA Document and I hereby sign the CLA