Biggest UI refresh since Oriath's End

[canuysal]

Fixes # .

Excuse the PR title, just wanted to hype a bit.

Description of the problem being solved:

PoB currently has a look reminiscent of a 90's advanced calculator. That reflects the soul of
the app, but it also adds to the complexity. This PR tries to solve that by refreshing the UI
components with modern looking variants, and by doing so also makes the app more appealing to
newer players.

UI is subjective, which is why I went with ShadCN style components, the style ~90% of the web
uses nowadays, so it's something we're all already used to. One thing I'm not sure about is
the contrast, so let me know if anything is hard to read.

The important part is that this is not a pile of one-off tweaks. Every base control used to
hardcode its own draw colours, repeating the same 1px light grey outline over a black fill in
about forty places. This adds src/Modules/UI.lua, a single module holding the colour tokens
plus rounded rectangle helpers, and points the controls at it. All the visual tuning now lives
in one token table, so if the contrast or the rounding needs adjusting, it's a one-file change
rather than a hunt through every control.

Two small assets come with it (908 bytes total). SimpleGraphic can only draw axis aligned quads,
so ui_round.png is an antialiased disc whose four quadrants act as rounded corners, and
ui_check.png is the check mark. Untextured geometry gets no antialiasing, which was very
visible on a diagonal shape once the tick sat on a filled box.

What changed, control by control:

• Buttons, drop downs, edits, check boxes: rounded corners, subtle borders, hover and
  pressed states that lift the fill instead of flashing a white outline. A ticked check box is
  now a filled box with a dark tick, like ShadCN's.
• Scroll bars: rebuilt. The stepper arrow buttons are gone, so the knob runs the full length
  of the bar. It's a thin rail at rest, fattens while you're using it, and hides entirely when
  there's nothing to scroll. Clicking the track pages toward the cursor.
• Side bar stat list: label and value are pinned to opposite edges instead of meeting in the
  middle, each run of stats between spacers is grouped into a card with hairlines between its
  rows, and rows have breathing room. Long labels are clipped so they can't run underneath their
  own value (they used to clip off the left edge of the panel instead). No stats were added,
  removed or reordered. Only how the existing list is drawn changed.
• Main skill section: it was built from 18px controls with 2px gaps, and a drop down draws
  its text at height minus four, so that meant 14px text sitting between the 16px label above
  and the 16px stat rows below. Everything in the section is now 20px, and drop down / edit text
  is inset from the left border rather than jammed against it.
• Top bar: the point display and build name are read-only, so they're framed like disabled
  buttons to match the rest of the bar. The 4px near-white rule under the top bar is now a
  hairline, and the vertical tick at the centre is gone. It marked where the bar's two anchor
  groups meet, which isn't information a user needs.
• Sections, popups, lists: same rounding and border treatment, filled selection rows instead
  of white-outlined ones.

Worth noting on contrast: the first pass lifted control interiors to around 10% grey and it
flattened the game's own text palette, since item rarity colours and the dimmer greys are all
mixed for contrast against black. Anything holding text now sits near black again and leans on
its border for definition. That's the part I'd most like a second opinion on.

Steps taken to verify a working solution:

• Ran the app and went through every tab, checking the controls in each: Tree, Skills, Items,
  Calcs, Configuration, Notes, Import/Export, Party, Compare.
• Checked every base control in each state: hover, pressed, disabled, focused, and drop downs in
  both dropped directions (up and down), with and without a scroll bar.
• Verified the side bar stat list against the previous build: same stats, same order, same
  values, including minion stats and the multi-line skill DPS entries.
• Confirmed scroll bars still work for both axes: knob drag, click-to-page, hold to repeat,
  mouse wheel, and the auto-hide when content fits.
• luajit HeadlessWrapper.lua loads clean.

Link to a build that showcases this PR:

Before screenshot:

After screenshot:

[canuysal]

Re: the GC pressure concern raised on Discord, the colour tokens are packed 0xRRGGBB numbers
rather than {r, g, b} tables, and nothing on the draw path allocates. The token table was
already built once at load, but three controls really were creating a fresh table every frame
to wrap what their borderFunc returned; that's gone.