Align pbtech with phpBB 3.3.16 prosilver

[avandenberghe]

Goal

Update pbtech to track prosilver changes from phpBB 3.3.16 so the style passes Customisation Database validation.

Context

• pbtech currently declares phpbb_version = 3.3.15 in style.cfg and Requirements: phpBB 3.3.15 or higher in README.
• phpBB.com validator now requires 3.3.16+; the previous revision was rejected partly because pbtech is a minor-version behind upstream prosilver.
• Local install is on 3.3.13 — not directly upgradable to 3.3.16 (known install quirks), so the diff has to come from upstream rather than from a working install.

Approach

1. Fetch the upstream diff: https://github.com/phpbb/phpbb/compare/release-3.3.15...release-3.3.16, filter to phpBB/styles/prosilver/.
2. For each changed prosilver template/CSS file:
   • If pbtech has its own copy (override), port the upstream change into pbtech's copy.
   • If pbtech inherits prosilver's version, no action needed (the parent style picks up the new version automatically when the install upgrades).
3. Update style.cfg: phpbb_version = 3.3.16.
4. Update README requirement: phpBB 3.3.16 or higher.
5. Bump style version (3.0.21).
6. Smoke-test locally on 3.3.13 (fine — pbtech still works on older phpBB; we're just declaring tested-against 3.3.16).
7. Visual sweep: index, viewforum, viewtopic, posting, UCP, ACP, search results, responsive widths.

Out of scope

• Single-file / hybrid CSS refactor — tracked in Adopt PBWoW-style hybrid CSS architecture for consistency #5.
• Anything related to phpBB 4.0 prep — separate concern.

Acceptance criteria

• Every prosilver template/CSS change between 3.3.15 and 3.3.16 either ported into the corresponding pbtech override or confirmed unaffected.
• style.cfg and README.md declare 3.3.16.
• Style version bumped (3.0.21).
• No visual regressions vs current 3.0.20.
• phpBB.com validator passes the resubmission.