Consolidate Jinja2/HTMX/Alpine.js templating stack across entire app

Jinja2 — single shared instance - All 11 UI route modules (ui, ui_blame, ui_forks, ui_topics, ui_notifications, ui_milestones, ui_stash, ui_emotion_diff, ui_collaborators, ui_settings, ui_similarity) now import from _templates.py instead of constructing their own Jinja2Templates instance — some were missing filter registration entirely - _templates.py enhanced with global template variables: MUSE_VERSION and now() available in every template without passing via ctx - jinja2_filters.py docstring updated to reflect centralized pattern

HTMX — extensions wired up - Downloaded htmx-ext-json-enc@2.0.1 and htmx-ext-response-targets@2.0.2 to static/vendor/ (json-enc was referenced in pr_detail.html but the file was missing — now fixed) - Both extensions loaded in base.html for all pages - response-targets enabled globally via hx-ext="response-targets" on the root container, enabling hx-target-4xx / hx-target-5xx error targeting on any form or HTMX request across the app