feat(chat): restyle transcript — left-aligned, bubbleless assistant, tool cards

All messages flow left in one column via the DaisyUI chat-start grid:
- user keeps a primary speech bubble;
- assistant/system/error render in a transparent (invisible) chat-bubble so
  they read as plain prose yet inherit identical left spacing, capped to a
  readable max-w-5xl column;
- tool call/result render as regular (non-speech) rounded cards, nested in the
  same grid so they line up too;
- role header labels dropped; chat-wide left padding added.
Alignment uses specificity-based variants (no !important).

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