fix(styles): import Tailwind's theme layer so spacing utilities work

[michaelpporter]

Fixes the root cause behind #736 and removes the workarounds it had accumulated. No issue number — this was never filed.

Stacked on #739 (→ #738 → #737). Merge in that order.

The bug

src/styles.css imported only tailwindcss/utilities. In Tailwind v4 the numeric --spacing scale lives in the theme layer, so every utility resolving against it had nothing to resolve to and was dropped at build time — 39 distinct classes across 15 files, silently.

Non-spacing utilities (.absolute, .flex, .items-center) compiled fine, which is exactly why this survived so long: most layouts still looked roughly right, just missing every gap, pad and offset.

The fix

@import "tailwindcss/theme" layer(theme);
@import "tailwindcss/utilities";

Deliberately not @import "tailwindcss" — the full bundle also pulls in Preflight, a browser reset that would fight Obsidian's own app styling.

Verified the blast radius

• All 39 utilities now appear in the compiled styles.css (checked programmatically against the built file, not the templates).
• The two global rules Tailwind emits — :root,:host and *,::before,::after,::backdrop — declare custom properties exclusively, zero visual properties. Nothing leaks into Obsidian's styling.
• Theme is tree-shaken to the 10 variables actually used. styles.css grows 5.4K → 8.1K.
• Largest newly-live values are w-48/w-60/h-32 on settings controls (intended sizing that was missing) and scroll-mt-40, which only affects scrollIntoView anchoring — no layout shift.

Expect a real visual diff: slightly roomier spacing in the settings tab and side views, and correctly positioned floating buttons in codeblocks. It should be an improvement everywhere, but it is broad — worth an eyeball before release.

Workarounds removed

One missing import had produced three separate coping strategies in src/components/codeblocks/:

File	Workaround	Now
CodeblockTree	hand-written .BC-codeblock-tree-{buttons,content} CSS keyed to Obsidian's --size-4-* vars (added in #739)	top-2 right-2 z-10 / pr-10
CodeblockMarkmap	inline style="z-index: 1; bottom: 0.5rem; right: 0.5rem" on the zoom toolbar	bottom-2 right-2 z-10
CodeblockMermaid + CodeblockMarkmap	copy buttons whose left-2 top-2 never applied — landing top-left by accident — and which disagreed on z-index	left-2 top-2 z-10, applied for real

All three codeblocks now position buttons through the same mechanism. Mermaid and Markmap keep their long-standing top-left placement (now with the intended 8px offset); only the tree moved, per #736.

bun run build && bun run test pass (379 tests, 0 type errors).