admin: responsive UI for phones, tablets and ultrawide

[Postlog]

Makes the admin panel usable on every screen size — mini-phones (320px) → phones → tablets → laptops → desktops → ultrawide. Almost all of it is static (internal/handlers/web/static/{app.css,app.js,index.html}); the only Go change is a one-line dev tweak (see "Dev" below). Desktop (≥992px) is visually unchanged. Rebased onto the current main (English UI #117 + the subscription-links popup #116); the new sublinks modal reuses the generic responsive modal.

What changed

• Navbar collapses into a hamburger below lg (992px): an animated toggle + a full-width collapsible menu. The tabs moved from <a> (no href, unreachable by keyboard) to <button> + a :focus-visible ring.
• Users/Nodes tables turn into cards with data-label rows below 992px (two-up on tablets via CSS-grid; the table returns at ≥992px). The user description shows inline on the card (not a floating gray tooltip), and Bootstrap's row-hover cell tint (an inset box-shadow that lit the card title block) is dropped on cards.
• Config constructors and rule/group/sub-rule rows stack below 768px; the orphaned delete ✕ is right-aligned.
• Touch: bigger tap targets on pointer:coarse; inputs keep their normal size (no 16px iOS-no-zoom bump — it blew the dense config selects/inputs out of proportion). Hover popovers open on tap (:focus-within + tabindex).
• Modals become a flex column that scrolls only the body — header/footer stay pinned even on a short landscape screen (max-height + dvh with a vh fallback).
• Monaco (base-YAML) no longer traps page scroll on touch.
• Ultrawide: the container widens (≥1600px → 1320px, ≥2200px → 1560px).
• Dev: render.go serves the live SUBGEN_STATIC_DIR assets with Cache-Control: no-cache, so an edited CSS/JS shows on a plain reload (heuristic caching otherwise served a stale asset for minutes — defeating the live-edit feature).

Verification

Driven with a real browser at 320 / 360 / 500 / 768 / 850 / 1024 / 1280 / 1600px (all tabs, modals incl. the subscription-links popup, login, plus a real :hover on the cards) — no horizontal overflow, no desktop regression. make all is green (build + vet + lint 0 issues + unit -race + integration + apitest). The Cyrillic guard (grep -rI '[А-Яа-я]') stays clean.

Commit authored by the bot postlog-vibecoder; the PR is opened under Postlog because the bot is currently rate-limited (spam mute). Awaiting a reviewer approval before merge.

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