Refactor metadata templates and update page titles/descriptions

[tom2drum]

Description and Related Issue(s)

Refactors the metadata generation system and updates page-level SEO copy. Page <h1> titles no longer include the chain name (that context moves to server-rendered meta titles); per-route descriptions are more specific; and the template system is consolidated into a single map with optional enhanced variants.

Proposed Changes

• Unified metadata templates — Merged title.ts and description.ts into templates/index.ts with per-route default and optional enhanced title/description templates.
• compileValue enhancement — Accepts { default, enhanced? } objects and uses the enhanced template when all its placeholders are populated from API data.
• Placeholder renames — %network_name% → %chain_name%, %network_title% → %chain_explorer_title%, %network_gwei% → %gwei_name%, %id_cap% → %id_formatted%.
• Page-specific descriptions — Replaced the generic default description on list/index pages (e.g. /txs, /blocks, /gas-tracker) with route-specific copy.
• Page title simplification — Removed conditional chain-prefixed <PageTitle> titles across index pages (transactions, tokens, gas tracker, user ops, etc.); chain context is handled in meta titles.
• SEO enhanced data scope — NEXT_PUBLIC_SEO_ENHANCED_DATA_ENABLED now only pre-renders meta titles server-side; removed h1 injection behavior from page components. Gas tracker FAQ is always shown.
• Tests — Expanded metadata snapshot tests for address and stats detail routes with/without enhanced data.

Environment variable changes

• NEXT_PUBLIC_OG_ENHANCED_DATA_ENABLED - Changed default value from false to true

Documentation-only updates in docs/ENVS.md (no validator changes):

• NEXT_PUBLIC_OG_DESCRIPTION — Example placeholder updated from %network_title% to %chain_name% to match the new template parameter naming.
• NEXT_PUBLIC_SEO_ENHANCED_DATA_ENABLED — Description updated to reflect that the flag only pre-renders page titles server-side (h1 injection removed).

Checklist for PR author

• I have tested these changes locally.
• I added tests to cover any new functionality, following this guide
• Whenever I fix a bug, I include a regression test to ensure that the bug does not reappear silently.
• If I have added a feature or functionality that is not privacy-compliant (e.g., tracking, analytics, third-party services), I have disabled it for private mode.
• If I have added, changed, renamed, or removed an environment variable
  • I updated the list of environment variables in the documentation
  • I made the necessary changes to the validator script according to the guide
  • I added "ENVs" label to this pull request

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