fix: larger, centered, higher-contrast tablet player controls (#97) - #132

on bright content (#97)

Reported: "On Android tablet the player controls are so small and on the
far left of the screen. It would be better to have them bigger and
centralized. Also if the video is bright white, it's very difficult to
see the controls, it would be nice to darken the gradient."

Three separate sub-bugs in PlayerScreen.kt around the bottom controls row:

1. Button sizes used `if (isTouchDevice) ... else ...` which collapsed
   phone and tablet into one branch. That branch used SMALLER buttons
   than TV (24 dp vs 28 dp) \u2014 backwards for tablet where viewing
   distance is short and finger targets must hit Material's 48 dp
   minimum. Tablets inherited the phone sizing and looked cramped.
2. `horizontalArrangement = Arrangement.Start` left-aligned the row on
   every device. TV remotes tolerate left alignment but on a 10"+
   tablet the icons cluster in the bottom-left corner with empty space
   to the right \u2014 exactly the "far left of the screen" complaint.
3. The bottom gradient peaked at `Color.Black.copy(alpha = 0.7f)` at
   the very bottom and faded out to 30% up. On bright white content
   the icons at the top of the control column sat on roughly 20%
   alpha, making them nearly invisible.

Changes (all scoped to the single bottom-controls Column, #97 only):

- Compute `isTablet` / `isPhone` from `LocalDeviceType.current` alongside
  the existing `isTouchDevice`, so the three device categories can be
  distinguished.
- Replace each button-size / icon-size / gap `if (isTouchDevice)`
  expression with a three-way `when` that gives tablets the LARGEST
  sizes (36-48 dp button, 22-30 dp icon), phones the compact sizes
  (24-34 dp button, 17-26 dp icon), and TV the original medium sizes
  unchanged. This makes tablet controls meet Material's 48 dp touch
  target minimum.
- Center the icon row on tablet (`Arrangement.Center`) instead of
  left-aligning. Phone keeps left-alignment because vertical orientation
  has limited horizontal space. TV keeps left-alignment because D-pad
  focus traversal assumes it.
- Strengthen the bottom gradient for touch devices only: starts at
  Color.Black 50% alpha at 20% and peaks at 85% alpha at the bottom.
  The old 70% peak wasn't enough on bright content; the new value makes
  icons readable on white backgrounds without being opaque enough to
  obscure the video entirely. TV keeps the lighter original gradient
  because TV content is usually not as bright as mobile photo/app
  backgrounds.

No behavioral or focus-traversal changes \u2014 buttons still respond to
the same D-pad / touch events, same focus requesters, same actions.

Closes #97