fix: terminal sized incorrectly on first load until window resize

[jolly-exe]

Problem

On first opening a ttyd session, the terminal doesn't fill its container —
there's dead space on the right/bottom edges — until the browser window is
resized, which silently corrects it.

Root cause

Xterm.open() calls fitAddon.fit() right after terminal.open(), while
xterm's default dom renderer is active. The SET_PREFERENCES message that
follows switches the renderer to the default webgl, which measures cell
size slightly differently (e.g. 7px vs 7.8px cell width in one repro) — but
nothing re-fits after that renderer swap, so cols/rows stay pinned to the
stale dom-renderer measurement. Any subsequent window resize triggers a
fresh fit() under the correct renderer, masking the bug.

Fix

applyPreferences() now calls fitAddon.fit() once after all preferences
(including rendererType) are applied, replacing the narrower font-only
refit that missed the renderer case.

Also fixes an unrelated pre-existing build break: preact's RefCallback
type now allows an optional cleanup return (void | (() => void)), so the
bare assignment in terminal/index.tsx's ref callback no longer type-checks
against current preact and blocks npm run build. Wrapped it to return
void. src/html.h is regenerated from the fixed frontend.

Verification

Built the actual production bundle and the ttyd C binary, served it against
a mock of ttyd's WS handshake (/token, /ws, SET_WINDOW_TITLE,
SET_PREFERENCES) in headless Chromium at 1000×700:

• Before fix: loads at cols=125; dispatching a same-viewport resize
  event jumps it to cols=135 — reproduces the reported bug exactly.
• After fix: loads at cols=135 immediately; a same-viewport resize is a
  no-op (matches the previously resize-only-reachable state).

npx tsc --noEmit and npm run build both pass cleanly.