The recent "pickers with previews" overhaul left the picker
sizing/presentation API spread across several overlapping knobs that
every call site had to set correctly — and many didn't, causing pickers
across the app to render at the wrong width, lose their container, or
stop dismissing. This PR consolidates that surface into a small,
hard-to-misuse API and makes correct sizing the default.

Net effect: a plain `Picker::uniform_list(delegate, …)` now renders
correctly out of the box (standard width, standard max-height, shrinks
to fit, dismisses properly), and the ~35 call sites only specify what
genuinely differs.

## API changes

**Presentation** — three overlapping booleans (`is_modal`, `is_popover`,
`is_resizable`) collapsed into one enum, with resizability living inside
the only variant where it's meaningful:

```rust
enum Presentation {
    Modal { resizable: bool }, // own chrome, dismisses on blur, optionally resizable
    Popover,                   // own chrome, dismisses on blur, never resizable
    Embedded,                  // host container owns chrome + dismissal
}
```

- `modal(bool)` is **removed** in favor of explicit, self-documenting
builders:
  - *(default)* → `Modal` (resizable iff it has a preview)
  - `.popover()` → `Popover` (menu-attached surfaces)
  - `.embedded()` → `Embedded` (pickers nested in a larger modal/view)
- Dynamic callers use `.when(cond, Picker::embedded)` (added `impl
FluentBuilder for Picker`).

**Sizing** — preview-vs-not now drives everything; the manual padding
knob is gone:

| Before | After |
|---|---|
| `vertical_padding` field + `no_vertical_padding()` | removed — derived
from whether a preview is visible |
| `height(...)` (ambiguous: fixed vs max) | `max_height(...)` (plain
pickers shrink-to-fit, capped here) |
| `minimum_results_width(...)` | removed — a plain picker's min width
tracks its opening width; preview pickers use standard internal pane
mins |
| default size = 60% viewport | default = `DEFAULT_MODAL_WIDTH` (34rem)
× `DEFAULT_MODAL_MAX_HEIGHT` (24rem, max) |
| resize handles gated on `is_modal` | gated on `is_resizable` (new
`resizable(bool)` builder; auto-`true` for preview pickers) |

Call sites now only override the exceptions: narrow popover selectors
(`initial_width`), the taller outline view (`max_height`), and preview
pickers (constructed via `*_with_preview`).

## Behavior fixes

- **Wrong widths everywhere**: pickers were falling back to
60%-of-viewport because the original migration set
`minimum_results_width` but never `initial_width`. Fixed at the source
via the new defaults.
- **Popovers had no container and wouldn't dismiss**: `is_modal=false`
was suppressing both the elevated background *and* blur-dismiss. Split
out so popovers keep their chrome and dismiss on click-away/escape. This
fixed the agent-panel model/profile selectors, sidebar recent projects,
and the settings theme/font/icon/ollama pickers (which were incorrectly
using `modal(false)`).
- **Sidebar recent projects stretched to full height**: was missing the
shrink-to-fit behavior; now capped and content-sized like other
popovers.
- **Preview crash**: removed an over-strict `debug_assert!` that
panicked when previewing an empty file (`message == None && editor
empty` is valid).
- **Preview-aware default size**: pickers open at standard width with
the preview hidden, and expand to the larger "telescope" size when a
preview is shown. Fixes the text finder rendering super-wide by default,
and makes the file finder expand (rather than cram its results) when you
toggle the preview.

---

Release Notes:

- N/A