Stop notes inventing a colour, and mark system prompts by shape

[dovvnloading]

Problem

Every note rendered a saturated green in an otherwise monochrome canvas, and a system-prompt note rendered green with a purple border.

The green was invented by the save path. SceneNode.color's own contract is "None means use the kind's own default colour, a rendering fallback that is entirely the frontend's job", and NoteNodeView already honours it (backgroundColor: data.color ?? undefined). But chat_library.py wrote str(note.get("color") or "#4a7c59") on every save, so:

• a note nobody ever coloured came back from a save/reload permanently green, and
• #4a7c59 is not one of GroupColorPicker's eight swatches (its Green is #3f8f5c), so that colour could never be deliberately chosen, nor re-chosen after changing it.

The notes.color column is TEXT NOT NULL, which is why a value was being invented at all.

The purple was a semantic marker borrowing a user-choice swatch. NOTE_SYSTEM_PROMPT_BORDER_COLOR was defined as GROUP_NAMED_COLORS[2].hex - the picker's own "Purple" - and applied as an inline borderColor. That put a semantic marker (this note governs the branch) and a user's colour choice in the same visual channel, indistinguishable the moment anyone picks Purple for a note body, and spent a hue in a palette whose whole design is monochrome. The meaning was already carried by shape: .note-node.system-prompt's dashed, heavier border plus the header's own badge.

Change

• The save path stores "" - the storable spelling of "no colour chosen" under a NOT NULL column - instead of a hex, and _legacy_default_to_none maps both "" and the old forced default back to None on read.
• Rows already carrying #4a7c59 are normalised on load, not rewritten, so existing databases render neutral with no destructive migration. A colour the user genuinely chose still round-trips as that hex.
• The system-prompt border loses its inline override and takes a theme token in CSS; the dashed 2px border and the badge continue to carry the meaning. NOTE_SYSTEM_PROMPT_BORDER_COLOR is removed.

Test plan

• Full backend suite on a clean checkout of this branch: 3095 passed, 19 skipped. ruff clean. npm run check green.
• New round-trip coverage in test_session_format_adr009.py, all reloading through the notes table exactly as loadChat does: an uncoloured note stays None across a full DB round trip; a chosen #3f8f5c survives unchanged; and a legacy row carrying #4a7c59 loads as None.
• NoteNodeView.test.tsx asserts a system-prompt note carries no inline borderColor, so the marker cannot drift back onto a palette hue.
• Verified live against the built bundle at the app's real 1440x900 window: the note renders rgb(37, 37, 37) (the shared node surface) with a dashed rgb(148, 148, 148) border and no inline styles at all.