feat: Google Drive downloads and JSON-embedded media scanning

[xAlcahest]

Summary

flux <google-drive-link> now downloads public Drive files directly: the bare link is rewritten to the drive.usercontent.google.com/download endpoint, which serves the file with HTTP range support, so flux's multi-connection accelerator applies (it bypasses Drive's per-connection throttle, ~190x with -n 16) and names the file from Content-Disposition. The page scanner also recognises Drive open?id=/file/d/ links and, when a page lists several, emits a list config that resolves each to its download URL. Separately, sites that embed episode media as escaped JSON in an HTML attribute (AnimeUnity-style: " quotes, \/ slashes) now scan correctly: extractor_resolve_url un-escapes \/, the media-URL capture stops at " in the path, and a media-list series emits a list over the embedded URLs.

This also fixes a series-ordering regression (the episode sort no longer reads digits out of opaque episode ids, so AnimeWorld keeps page order while kissanime /episode-N/ sorts 1..N), stops a no-media skeleton from being stashed as an unparseable --save-config, and updates configure.ac for gettext >= 0.20 so the project builds on current systems.

Changes

• src/scan.c: recognise Google Drive media links and emit a GDrive list config matching the page's open?id= or file/d/ form; detect a media-list series from direct candidates; un-escape JSON in captured URLs; stop the catch-all media capture at " in the path while keeping multi-param query strings; skip and cap the size-probe so scans stay fast.
• src/extractor.c: extractor_resolve_url un-escapes JSON \/ so escaped absolute URLs resolve.
• src/text.c: gdrive_normalize() rewrites a bare Drive link to its direct-download URL on the download paths; send a browser User-Agent for Drive unless -U was given; episode_number() recognises only episode/ep tokens and numeric segments (opaque ids ignored); a pure no-media skeleton is no longer stashed for --save-config.
• src/scan.h, src/conf.h: media-list series fields; ua_explicit flag.
• src/test_scan.c, src/test_extractor.c: AnimeUnity, GDrive (open?id= and file/d/), signed-URL, episode-order, and gdrive_normalize cases.
• configure.ac: AM_GNU_GETTEXT_VERSION -> AM_GNU_GETTEXT_REQUIRE_VERSION([0.21]).