feat: redesign login and retain view transitions

[wibus-wee]

Related issue

Problem / pressure

The login page presented one provider as the default choice and changing between email sign-in/sign-up could jump as the retained form changed height. Returning users also had no local reminder of the sign-in method they last completed.

Summary

• Redesign login around the theme-aware dither visual and equal provider choices.
• Remember only the last successfully completed login method on this device.
• Keep OAuth and email panels mounted and animate their slide and intrinsic height together.
• Add browser regressions for retained form state, input identity, resize, and reduced motion.

Visual explanation

flowchart TD
  LoginState[Login state and drafts] --> Slot[LoginViewSlot]
  Slot --> OAuth[Mounted OAuth panel]
  Slot --> Email[Mounted email panel and retained form]
  LoginState --> Mark[Theme-aware dither visual]
  Success[Successful sign-in] --> Preference[Device-local method preference]

Loading

Before / after

Before	After
Centered login card with a visually favored provider	Inset visual and equal provider choices
Form content rebased/jumped during mode changes	Mounted panels slide while measured height follows continuously

Test plan

• git diff --check: passed.
• Oxfmt check: 9 code/config files passed.
• Automated and browser tests were not rerun during the split.
• pnpm run docs check still reports oversized component guidance files and existing links into absent Kimi/Pi submodules.

[github-actions]

Desktop PR smoke regression failed on commit 95da154c7589d043c656c090153689db44e521a6.

• Workflow run: https://github.com/LodyAI/Lody/actions/runs/39211844029
• Failed scenarios indexed: 0
• Recording: 1/1

Recordings not attached

• Run evidence: failure-index.json is missing

The Actions artifact retains the complete trace, screenshots, logs, and runtime evidence.

[github-actions]

Desktop PR smoke regression failed on commit 78a160cc5bfd31b5f97ebd788050770293ab33ab.

• Workflow run: https://github.com/LodyAI/Lody/actions/runs/39221734901
• Failed scenarios indexed: 0
• Recording: 1/1

Recordings not attached

• Run evidence: failure-index.json is missing

The Actions artifact retains the complete trace, screenshots, logs, and runtime evidence.