e2e: hoist login/installApp/reloadBackend into helpers.ts and harden shared assertions

[gonzalesedwin1123]

The three Playwright specs under e2e/tests/ each carry their own near-verbatim copy of login() and installApp() (01-spp-starter-spmis.spec.ts, 02-spp-starter-farmer-registry.spec.ts, 03-spp-dci-compliance.spec.ts; spec 03 also adds reloadBackend()). Spec 01's login() additionally handles Odoo's avatar picker that appears once two accounts have logged in. A fourth spec would mean a fourth copy.

Proposed:

• Move login, installApp and reloadBackend next to resetStack() in e2e/tests/helpers.ts, keeping spec 01's avatar-picker handling as the single implementation.
• Fix the comment in installApp ("Remove all preset filter chips (there are two by default)") which contradicts the single .click() the code performs; Playwright strict mode would throw if two matched.
• Reuse the pageerror collector introduced in spec 03 (uncaught errors during webclient mount) in the other two specs; it is the assertion that catches a component throwing at setup() (useService("rpc") no longer exists on Odoo 19 - latent crash in spp_change_request_v2 review panel and spp_dci_compliance security warning #450), which a blank page otherwise hides.
• Spec 02 predates the "Adding new tests" rule in e2e/README.md and lacks the header comment block; add it.
• Optional coverage for spec 03: the zero-warning path (turn the last two settings off, assert the item disappears) and clicking "View DCI Settings" to assert the System Parameters list opens with the dci.* rows.

Refs: #442 (e2e suite), #527 (spec 03).