perf: fix slow page loads and gate performance in CI

- Ship one hashed Tailwind stylesheet instead of two (one render-blocking)
- Compress responses in-app (brotli/gzip, streaming-safe) and precompress
  the node build so local and CI measurements match production
- Validate the Supabase session once per request
- Render the homepage immediately and stream public stats
- Stream a Pokedex's entries with the page instead of fetching after
  hydration, running the rows and count queries in parallel
- Shrink the avatar and offline placeholder images, fix layout shift,
  contrast, link names and missing meta descriptions
- Add Lighthouse CI (mobile + desktop) with score and metric budgets,
  bundle-size budgets in the build tests, and signed-in speed scenarios

tests/bdd/features/performance.feature

@@ -0,0 +1,19 @@
+Feature: Signed-in page speed
+  As a trainer
+  I want my Pokédexes to open and switch quickly
+  So that tracking catches never feels sluggish
+
+  Lighthouse CI covers the public pages; these budgets cover the signed-in ones it can't reach.
+
+  Background:
+    Given I am signed in
+    And I have a Living Dex named "Speed Check"
+
+  Scenario: A Pokédex opens without a second round trip for its entries
+    When I load the Pokédex page directly
+    Then its entries appear within 5 seconds
+    And the browser did not request the entries separately
+
+  Scenario: Moving between my Pokédex list and a Pokédex is quick
+    When I switch between my Pokédex list and the Pokédex
+    Then each switch finishes within 3 seconds