Add Gemini to the AI connector modal and the docs

[dimitrieh]

What

Adds Gemini to the AI connector modal from #8483, and the matching section to the third-party-agents docs.

The modal shipped with Copilot, ChatGPT, Claude and Local, matching the widget on flowfuse.com/ai at the time. Neither offered Gemini.

• Gemini reaches a custom MCP server through Settings & help → Connected Apps → Add a custom app. The platform supports Dynamic Client Registration, so there is no client ID or secret to enter.
• Google limits custom apps to Gemini Spark on personal accounts in the US. Both the tab and the docs say so, the way the ChatGPT tab says developer mode needs a paid plan.
• gemini.svg is the same mark the website carries, copied in because the modal has its own icon directory.

The docs page embeds the website's agent-setup-tabs component, so its Gemini tab appears as soon as FlowFuse/website#5799 lands. The prose section keeps "Where to add it, per agent" from listing fewer agents than the tabs above it offer.

Testing

Checked statically, not by running the app:

• CLIENTS parses and every entry has a name, a mark, a step title and a URL.
• The SVG is well formed and its gradient reference resolves inside the file, so it renders standalone as an <img>.
• The new import sits in path-alphabetical order with the existing three.
• Not verified: the modal rendered in a running platform. Worth a click-through on the preview: five tabs, Gemini between Claude and Local, the sparkle at tab size, and the copy-URL button still working from that tab.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.92%. Comparing base (223aa66) to head (46097cb).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8492      +/-   ##
==========================================
- Coverage   76.92%   76.92%   -0.01%     
==========================================
  Files         460      460              
  Lines       24781    24782       +1     
  Branches     6610     6610              
==========================================
  Hits        19064    19064              
- Misses       5717     5718       +1     

Flag	Coverage Δ
backend	76.92% <ø> (-0.01%)	⬇️

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[dimitrieh]

Updated. This now builds on #8499, because the two collided in two places and merging them in either order went wrong on its own.

1. The modal client list auto-merged with no conflict but put Gemini after the two coding agents, since both branches insert immediately before the Local tab. Gemini is anchored on the ChatGPT entry now: claude copilot chatgpt gemini claude-code codex local.
2. The docs did conflict: this branch adds a Gemini section directly above Command-line and editor agents, and Add Claude Code and Codex to the AI connector modal and the docs #8499 renames that heading to Coding agents. Resolved keeping both, in reading order.

Worth noting GitHub reported both PRs as MERGEABLE throughout, because it only tests each against main, never against its sibling.

Merge #8499 first.

[dimitrieh]

converting this to a draft for now until decision has been reached on supporting gemini officially as only gemini spark users could use this.